1. Field of the Invention
The invention relates to a heat conductor coil for heating a flowing gaseous medium, primarily air, as well as an electric resistance heating element with a heat conductor coil fastened with intermediate positioning of securing elements on a support plate.
2. Description of the Related Art
Coil-shaped, but also zigzag-shaped or meander-shaped, heat conductors are employed primarily as heating elements in electrical devices for heating air, for example, in hair dryers, heater fans, convection heaters or clothes dryers or similar applications. However, they can be used in the same way also for heating other gaseous media. In such devices, an airflow generated by a fan is guided through a heating member which is comprised of at least one such heating element. When the air flows through the heating element, heat exchange takes place between the heat conductor and the airflow, which heat exchange should occur as uniformly as possible and, if possible, without heat losses.
In most known heating elements the heat conductor is comprised of a coil-shaped or meander-shaped resistance wire which is connected directly on a support plate of electrically insulating and heat-resistant material such as, for example, Micanit (German patent documents DE 25 30 075 A1, DE 25 35 478 A1, DE-PS 29 44 132). These known heating elements have in common that the heating wire requires for its fixation direct contact with the support plate, and also that the type of connection of the heating wire on the support plate depends on the form of the heating element. Since the heating wire is supplied with electrical energy, the support plate not only must be electrically insulating but must also be comprised of a heat-resistant material, such as, for example, Micanit. As a result of the support plates positioned parallel to the flow direction, the flow cross-section is divided into individual chambers which impair a uniform heat exchange.
However, a heating element is already known in which the coil-shaped heat conductor is no longer directly fastened on the support plate but indirectly by means of securing elements which are connected on the support plate and on which, in turn, the heat conductor coils are fastened (DE 44 43 725 A1). This avoids, on the one hand, direct contact locations between the heat conductor and the support plates so that the durability of the support plates is extended. Moreover, this has the additional advantage that the flow cross-section is no longer divided by support plates which results in an improved heat dissipation and thus a greater efficiency.
In this known heating element, the securing elements for fixation of the heat conductor coils are comprised of a shaft having at one end a securing part for fixation of the coil and having at the other end a foot part for attachment on the support plate. The securing part forms a flat receptacle having a U-shaped undercut for a winding tip of a coil having a flat-oval up to a flat-rectangular cross-section. The foot part is comprised of two parallel legs, which project at a right angle from the shaft and have at their underside tabs which are pushed through slots in the support plate and are then bent.
This configuration of the heating element not only has the advantage that direct contact locations between the heat conductor and the support plate are prevented but also the additional advantage that, as a result of the securing elements, which can be very narrow in the flow direction, the heat conductor coil is freely positioned within the airflow so that a relatively beneficial heat exchange can be achieved.
It is an object of the present invention to provide a possibility for a further improvement of the heat utilization of such electrical resistance heating elements.
In accordance with the present invention, this is achieved in that the heat conductor forming the coil is deflected within the circumferential surface of the coil defined by the ascending line of the coil out of this ascending line alternatingly in opposite directions.
In regard to the use of such a heat conductor coil, the present invention is directed to employing such a coil in a heating element for heating a flowing gaseous medium, primarily air, wherein at least one coil with interposition of securing elements is fastened on a support plate.
Furthermore, the invention relates to an electrical resistance heating element in which the securing elements for attachment of the heat conductor coil on the support plate have on a shaft a securing part for securing a winding of the heat conductor coil and a foot part for attachment on the support plate, wherein the securing part has two arms on which a receptacle for attachment of the section of a winding of the heat conductor coil is formed, respectively.
The basic idea of the invention resides in that as a result of the configuration of the heat conductor coil, in particular, a coil configuration of a flat-rectangular cross-section, with undulations of a short wave length relative to the dimensions of the coil within the circumferential surface of the coil defined by the ascending line of the coil, the flow cross-section is penetrated by the heat conductor generating the heat with a very tight mesh. This results in multiple turbulences of the air flowing through a heating element provided with such a coil with the result that a substantially more beneficial heat transfer from the heat conductor onto the air flowing therethrough is realized. In this way, for the same heat output, a significant reduction of the energy consumption by 20 to 25 % is achieved.
Special advantages in this connection result when the heat conductor is comprised of a thermistor wire, i.e., of a material in which the electrical resistance changes with the temperature, in particular, of a thermistor material having a positive temperature coefficient (PTC).
A heat conductor coil embodied according to the invention can be used basically for any suitable heating element in which the coil is arranged transversely to the flow direction. However, special advantages result when using the coil in an electrical resistance heating element in which at least one coil is connected to a support plate with interposition of securing elements.
Functional and manufacturing advantages also result from the inventive configuration of the securing elements for the heat conductor coil, i.e., of the intermediate members between the heat conductor coil and the support plate which effect the substantially free suspension of the coil. With the special configuration of the securing part of this securing element which holds one winding of the coil on two oppositely positioned sections, a functional securing of the coil without deflection of individual windings and a safe fixation of the coil result. On the other hand, the foot part of the securing elements can be configured according to the corresponding configuration of the heating element. In this connection, it is possible to guide the heat conductor coil approximately in a circular ring shape about a central circular support plate wherein the securing elements are positioned in the plane of the support plate and of the coil, but also to arrange the securing elements at a right angle to a support plate in order to be able to position the coil in the corresponding direction.
A particularly advantageous embodiment of the heating element solves the problem of known securing elements which, as a result of their particular shape, have the tendency to retain dust particles and lint entrained in the gas flow. Over the course of time, this results in the collection of dust and lint which initially results in the surface area of the flow cross-section being reduced and the flow resistance being increased so that for the operation of the heating element more energy must be supplied.
More important however is the risk that the dust and lint deposits could ignite on the hot heat conductor thus causing a fire. In order to counteract this danger, it is therefore necessary to service and clean devices provided with known heating elements in regular intervals. This problem is of particular importance in regard to clothes dryers in which a more than proportional amount of lint is entrained in the gas flow.
The solution to this problem is realized with the invention with a securing element with the following features: the securing element is comprised of a tubular section with substantially closed mantle surface wherein one winding of the heat conductor coil can be inserted into the tubular section. In this connection it should be emphasized that such a securing element is not limited to heating elements with a heat conductor coil according to the invention. Instead, the securing elements according to the invention can be used in connection with all known heat conductors of the prior art which are to be fastened indirectly on a support element.